using prompts (delimited continuations) to build generators

generator.scm

(define yield (prompt-tag))

(define (count-helper)
  (let loop ((n 0))
    (abort-to-prompt yield n)
    (loop (1+ n))))

(define this-step count-helper)

(define (count)
  (call-with-prompt yield
    this-step
    (lambda (next-step result)
      (set! this-step next-step!)
      result)))